The present invention relates to a high luminance light emitting diode (LED) including an LED element, and to a method for manufacturing the LED, and more particularly to the LED which is improved in heat radiation thereof. The LED element of compound semiconductor is widely used because of long life and small size. Further, the LED element of GaN semiconductor which emits blue light has been produced, and the LED including this kind of LED element is used in color display devices also in a small color backlight system of the portable telephone and in an automotive display, and the utilization field of the LED is further expanded as an illumination device having a high luminance and high power.
In recent years, various LEDs of the surface mount type are produced because of mass productivity and miniaturization of the LEDs. However, when those kind of LEDs are operated at high luminance and high power, there is a problem of heat radiation. Namely, if the driving current is increased in order to increase the luminance, the loss of electric power increases in proportion to the increase of driving current, and most of electric energy is transformed into heat, thereby increasing the heat of the LED to high temperature. The light emitting efficiency (current-light transformation efficiency) of the LED decreases as the temperature of the LED is elevated. Further, the life of the LED element becomes short, and the transparency of the resin covering the LED element decreases because of color change thereof at high temperature, which causes the reliability of the LED to reduce.
In order to resolve these problems, various heat radiation means have been proposed. As one of the means, an LED is proposed, wherein a pair of conductive members made of heat conductive metal are secured to an insulation member, and an LED element is mounted on the conductive members. Japanese Patent Application Laid Open 11-307820 discloses this kind of LED.
FIG. 16 is a perspective view showing the conventional LED.
The LED 1 comprises a pair of conductive members 2a and 2b made of metal having high thermal conductivity, an insulation member 3 made of resin for insulating the conductive members 2a and 2b and combining the members. The insulation member 3 has an opening 3a having an elongated circular shape. A part of each of the conductive members 2a, 2b is exposed in the opening. An LED element 4 is secured to exposed parts of the conductive members 2a, 2b, so that the LED element 4 is electrically and thermally connected to conductive members 2a and 2b. The LED element 4 is encapsulated by a transparent sealing member 5.
The LED 1 is mounted on a print substrate 6, and the conductive members 2a and 2b are connected to a pair of conductive patterns 6a and 6b by solders. When driving current is applied to the LED element 4 from the patterns 6a and 6b through conductive members 2a and 2b, the LED element 4 emits light. Heat generated in the LED element 4 by power loss is transmitted to the print substrate 6 through the conductive members 2a and 2b, so that the heat is efficiently radiated from the print substrate 6 if the substrate is made of a material having high thermal conductivity.
Another conventional heat radiation means is disclosed in Japanese Patent Application Laid Open 2002-252373. In the means, a base for mounting an LED element and lead frames as terminal electrodes are made of same material, the base and the lead frames are positioned at the same level, and the base is directly mounted on a substrate.
FIG. 17 is a sectional view showing the conventional LED. The LED 10 comprises a base 11 and a pair of lead frames 12a and 12b which are made of same conductive material and securely mounted on a print substrate 16 by solders 17, so that the base 11 and lead frames 12a, 12b are positioned at the same level, and are thermally combined with each other. An LED element 13 is mounted on the bottom of the base 11, thereby to be thermally combined with the base 11.
The anode and cathode of the LED element 13 are electrically connected to the lead frames 12a, 12b by lead wires 14a and 14b. A transparent resin 15 encapsulates the LED element 13, lead frames 12a, 12b and wires 14a, 14b. When driving current is applied to the LED element 13 from the print substrate 16 through lead frames 12a and 12b, the LED element 13 emits light. Heat generated in the LED element 13 by power loss is transmitted to the print substrate 16 through the base 11, so that the heat is efficiently radiated from the print substrate 16 if the substrate is made of a material having high thermal conductivity.
As another means, there is proposed that through holes are formed in the print substrate 16 by conductive patterns, and heat radiation members are disposed on the underside of the print substrate, so that heat is transmitted to the heat radiation members.
In the LED shown in FIG. 16, if the print substrate 6 is made of a material having high thermal conductivity such as a metal core substrate, heat radiation effect is expectable.
However, the print substrate 6 is generally made of cheap material such as an epoxy resin having low thermal conductivity. Namely, the thermal conductivity of the epoxy resin is one several hundredth of copper alloy as the material of the metal core substrate. Therefore, the heat is not sufficiently transmitted to the print substrate, thereby raising the temperature of the LED element, and reducing the quality thereof.
However, metal core can not be used because of high manufacturing cost. Furthermore, there is a problem that since it is difficult to wire on both sides of metal core substrate, high density mounting is impossible. In addition, it is necessary to insulate the surface of the metal core substrate by providing an insulation layer on the substrate since the metal core is conductive material. However, the insulation layer reduces the thermal conductivity to decrease the heat radiation effect.
The LED 10 of FIG. 17 also has the same problems as the LED of FIG. 16. Since the base 11 is directly adhered to the print substrate 16, the thermal conductivity from the base to the print substrate 16 must be effective. However, if the print substrate 16 is made of epoxy resin, heat radiation effect can not be expected. Further, if the conductive through holes are provided between the base 11 and the heat radiation members secured to the underside of the print substrate 16, heat connection there-between is not so effective, and hence great heat radiation improvement can not be achieved.